In order to understand B cell function within the immune system of old animals, we plan to study the antigen-induced growth and differentiation of antigen-specific populations. Initially, the expansion of B cells to both thymus-independent (TI) and thymus-dependent (TD) antigens will be evaluated and correlated to differentiation of the activated cells to antibody-forming cells (AFC). These experiments will examine the ability of B cells from aging mice to proliferate to test the hypothesis that such cells are inefficient in providing suitable progeny following challenge by antigen. Secondly, the responsiveness of TD antigen activated B cells to nonspecific macrophage and T cell growth and differentiation cytokines will be evaluated. Decreased cytokine influences on B cells from old mice could be involved in the overall inability of these cells to expand following the initiation of the immune response. Thirdly, the ability of memory B cells from old mice to expand and provide sufficient progeny for the secondary immune response will be examined. The results of these experiments should contribute significantly to our understanding of B cell function in the aging immune system, independent of defects which may occur in the accessory cell populations.